U-M
scientists have developed a new generation of gutted
viral vectors that deliver the gene for dystrophin to the muscles of
adult mice with muscular dystrophy without triggering their immune
systems to attack the foreign virus. The vector is called a
gutted virus because it has been stripped of most of its
original genes to make room for the large dystrophin gene.

Dystrophin is a protein critical for normal maintenance of muscle
tissue. Muscular dystrophy is caused by mutations in a large, complex
gene which contains instructions telling muscle cells how to produce
dystrophin. Because they lack the genetic code to produce dystrophin,
children with muscular dystrophy gradually lose muscle tissue and die
of heart or respiratory failure.

At last years American Society for Human Genetics meeting,
Jeffrey S. Chamberlain, associate professor of human genetics,
reported the viral vectors ability to induce long-term
expression of the full-sized dystrophin protein in an
immuno-deficient strain of adult mice with Duchenne muscular
dystrophy. At this years meeting, Giovanni Salvatori,
post-doctoral research fellow in Chamberlains lab, reported the
same results in mice with a normal immune system.

Using this new version of our viral vector, we have induced
stable production of dystrophin for at least four months in muscle
fibers of adult, dystrophic mice with normal immune systems,
Salvatori said. Removing a reporter gene called LacZ from the
vector was the key to reducing the vectors strong immunogenic
effect. Although we still see a small immune response, it peaks after
30 days and does not appear to adversely affect the ability of mouse
muscle to take up the vector and produce dystrophin.

A new cell packaging line, also under development at the U-M, will
make it possible to produce large amounts of the vector without
contamination by other proteins or viruses that could trigger an
immune response in humans, according to Salvatori.

The result is significant because stability and immune response
were two major problems that remained to be solved in mice before the
U-Ms dystrophin vector could be tested for safety and
effectiveness in humans. Chamberlain and Jerry Mendel of the Ohio
State University Medical Center plan to begin testing the vector for
safety in humans in the spring.

Chamberlain and his research team have been overcoming technical
obstacles to an effective gene therapy treatment for muscular
dystrophy for eight years. They have focused on using modified
adenovirusesthe same type of virus that causes coldsas
delivery vehicles, because they have a natural ability to enter
muscle cells and deliver the dystrophin gene. Chamberlain is a member
of the Department of Human Genetics and is affiliated with the Center
for Gene Therapy in the Health System.

Current collaborators in the program include Ph.D. research
fellows Catherine Barjot, Catherine Begy, Christiana DelloRusso,
Dennis Hartigan-OConnor, Ann Saulino and Michael Hauser, who is
now at Duke University.

The work is funded by the National Institutes of Health, the
Muscular Dystrophy Association and a private foundation established
by a Birmingham, Mich., couple, Chip and Betsy Erwin, to support the
U-M research program.